Portable purification devices with serially arranged antimicrobial resins and air-tight membrane filters suffer from the fact that air may be trapped between the resin and the membrane which blocks for a proper flow of water through the system. For this reason, water purification systems have been developed with vent conduits that provide a passage into atmosphere from the volume between the resin and the membrane.
An example is disclosed in U.S. Pat. No. 6,454,941 for a gravity filter with a resin in a compartment above a membrane that formed as a vertically oriented cellular honey-comb structure. An air vent conduit extends from the volume between the resin and the filter membrane up to the upper edge of the resin below water inlet cap. Water in the device cannot by mistake be driven all the way up to the air vent conduit exit, because a goose-neck conduit for water consumption has its water exit below the air vent conduit exit, and water would leave that conduit first.
Another example is published in U.S. Pat. No. 6,638,426 for a gravity filter with a vent tube that is provided above a filter that has vertically arranged, hollow membrane fibres. The vent tube extends centrally through an upper resin chamber and ends higher than the top water inlet above the resin. The vent tube extends from a lower, tangential fluid divider, which has a form of an inverted funnel. The fluid divider has apertures for water to flow down from the resin through the apertures and into the housing with the filter.
These systems have dedicated means for venting the volume above the membranes. Other water purification systems are disclosed with vent tubes extending through an upper dirt water container to above the water level in the dirt water container. Examples are given in U.S. Pat. No. 4,749,484, U.S. Pat. No. 4,978,449, U.S. Pat. No. 4,759,474, U.S. Pat. No. 4,800,018, and EP404573. Alternative systems with vent tubes outside the dirt water container are disclosed in GB1112072, U.S. Pat. No. 3,709,362, EP245585, U.S. Pat. No. 5,281,309. Further vent systems are disclosed in DE3828008, U.S. Pat. No. 3,281,108, U.S. Pat. No. 6,159,363, U.S. Pat. No. 7,276,169. As it appears from these disclosures, portable systems have vent tubes extending through the upper dirt water container, whereas for stationary systems, vent tubes that imply air escape from the upstream side of the filter medium can extend through the upper dirt water container or extend outside the upper dirt water container. Thus, there seems to be a systematic limitation for portable devices in this respect, which is due to the desired compactness of these devices, although it implies some limitations for the possible design of the portable water filtration devices.
International patent application WO2008/110166 discloses in FIG. 7 a gravity driven water filtration system with an upper dirt water container connected by a tube to a filtration unit. In use, the filtration unit oriented vertically, and air from the filtration unit is exiting in an upward direction through the tube and into the dirt water container against the stream of dirt water from the container. In practice, this is no problem, although for start up, it is more convenient to open a forward flush valve in order to empty the device for air quickly.
If the filtration unit of the system disclosed in FIG. 7 of WO2008/110166 would have a horizontal orientation, air would be trapped in the filtration unit, which is also the reason, why the module has a vertical orientation when in use. However, even in a horizontal orientation, it would be possible to vent the system by opening a forward flush valve during feeding with dirt water from the opposite end. Once air is flushed out of the system, this flush valve could be closed again. An indication that the air is flushed out of the system is the appearance of water at the exit from the flush valve. Thus, also during proper operation in horizontal orientation, air trapping is no problem for operation of this device, in principle.
Due to the relatively easy venting of the system of WO2008/110166 by using the forward flush valve, such a horizontal orientation of the filtration device may be tempting to use in other filtration systems, if space and compactness is a critical issue. Problem occurs however, if such a system is not understood properly by the end user. For example, for the user not familiar with the system, the procedure for air venting may not be obvious, and the user may regard the system as not properly functioning simply due to the fact that air trapped in the system prevents a proper water flow from the clean water exit valve. Also, the forward flush valve may be mistaken as the clean water supply, and the user may consume dangerously contaminated water coming from the dirt water container without having passed the filter membrane. These risks may appear hypothetical at first glance; however, it should be pointed out that such filter systems are used in emergency areas with sometimes hectic activity where a proper training of the user is not always possible due to lack of resources or lack of time. In such situations, the risk for mistakes is high and may cause fatal results for the user.
The above described system also suffers from the fact that no proper clean water accumulator is provided, which implies a large risk for back-contamination, especially in emergency situations, where cleaning-water is scarce, and the focus is not especially on hygiene.
A portable water purification system for emergency situations is published on the Internet on http://espwaterproducts.com/emergency-preparedness-water-disaster-relief.htm. The system comprises a top dirt water container covered by a prefilter for removing debris and large sediment particles when water is added. The dirt water container contains an iodine tablet for destroying bacteria and virus. Inside the dirt water container, a vertically arranged sediment pre-filter is provided for removing contaminants down to 0.5 microns in size, including cysts. The lid of the clean water tank contains a filter connector that is directly connecting the outlet of the sediment filter to the inlet of a vertically arranged multimedia cartridge which removes industrial and agricultural contaminants as well as chlorine, iodine, and bad tastes and odours. The multimedia cartridge is provided below the lid and inside the clean water tank. The clean water tank is provided with a tap for dispensing of the purified water.
As the lid of the clean water tank is easily removable, contamination of purified water can easily become a problem if the lid is not properly placed, or it is removed in not sufficiently hygienic conditions. The latter is important when knowing that in emergency situations, but also in typical households in Africa's rural countryside, animals mix with humans and may get easily access to the purification system and its surroundings.
A further disadvantage of this system, as well as the systems disclosed in U.S. Pat. No. 6,454,941 and U.S. Pat. No. 6,638,426 is the fact that a continuous filling of water into the upstream tank leads to an overflow in any downstream part. Thus, the user has to keep track of how much water is in the tank upstream of the filter relatively to how much is in the downstream part in order not to waste water. The latter is an important issue in emergency situations, where water is scarce.
For these reason, there is an ongoing effort to improve portable water filtration systems in the direction of higher safety and user-friendliness, especially in connection with portable filters for use in emergency situations.